What Is Reverse Osmosis and How Does an RO Water Filter Work?
Article Summary
Reverse osmosis (RO) uses pressure to move water through a semipermeable membrane, reducing dissolved salts and other substances. The membrane separates treated water, called permeate, from reject water, called concentrate. Under-sink RO systems combine this process with sediment and carbon filters to supply a dedicated drinking-water faucet.
What brings you here? Choose a starting point, or keep reading to understand the process.
How Does Reverse Osmosis Work?
In a typical tank-based kitchen system, incoming water passes through prefilters before reaching the RO membrane. The membrane produces two streams: treated water for the faucet and concentrate for the drain. The prefilters and final carbon filter have different jobs from the membrane.
From the Cold-Water Line to Your Glass
01
Prefilter the incoming water
A sediment prefilter captures particles such as sand, silt and rust. A carbon prefilter reduces chlorine taste and odor and helps protect a chlorine-sensitive membrane.
02
Separate water at the RO membrane
Pressure moves water through the membrane. Many dissolved salts and other substances are rejected into the concentrate stream. Performance depends on the complete system and operating conditions.
03
Store treated water; send concentrate to the drain
A tank-based system stores permeate for later use. A separate drain line carries concentrate away during production. The storage tank holds water; it is not another filtration stage.
04
Finish at the drinking-water faucet
Water passes through a final carbon polishing filter on its way to the dedicated faucet. Some systems can also serve a refrigerator or ice maker when the installation and system specifications allow it.
The Science: Osmosis vs. Reverse Osmosis
How the RO Membrane Separates Water
Osmosis: water naturally moves across a semipermeable membrane toward the solution with a higher concentration of dissolved material.
Reverse osmosis: applied pressure moves water in the opposite direction. The membrane allows water to pass while rejecting many dissolved ions and other substances.
A home RO system needs feed-water pressure within its specified range. Temperature, source-water quality and membrane condition also affect production. Check your model’s manual rather than assuming every RO system has the same pressure requirement.
RO separation differs from a sediment cartridge that holds captured particles in its media. During RO production, flowing concentrate carries rejected material toward the drain. The system’s flow restrictor and valves support this operation; they are working components, not extra filtration stages.
What Is RO Water?
RO water is water treated by reverse osmosis. It generally has fewer dissolved minerals and salts than the incoming water. In a home system, carbon stages also help address chlorine taste and odor.
Does RO Remove Minerals?
RO reduces many dissolved minerals along with other dissolved substances. This can change taste and pH. Some systems add a remineralization stage for taste preferences.
The term “RO water” does not tell you every contaminant the system reduces. Check the exact model’s documentation and use water suitable for its specified application. A water report or appropriate testing helps you choose treatment for your actual concern.
What Are the Filtration Stages in an RO System?
Each filter has a different job. Here are the four common treatment roles in a tank-based under-sink reverse osmosis water filter.
01
Sediment Prefilter
Captures particles
A sediment cartridge captures particles such as sand, silt and rust before later stages.
02
Carbon Prefilter
Conditions the feed water
Carbon reduces chlorine taste and odor and helps protect chlorine-sensitive RO membranes.
03
RO Membrane
Reduces dissolved substances
The membrane separates treated water from concentrate and reduces many dissolved salts and other substances.
04
Post-Carbon Filter
Polishes taste and odor
A final carbon stage provides taste-and-odor polishing before the water reaches the drinking-water faucet.
Representative cartridge designs—not a replacement set for every RO system. Your model may use different housings, combine functions or place additional filters in the treatment path.
The Parts of a Tank-Based RO System
Follow the main water path through the sediment prefilter, carbon prefilter, RO membrane, storage tank and carbon postfilter. The four numbers identify treatment stages. Supporting valves and connections are not all shown; use your model’s manual for installation and service.
See What the System Components Do
| Component | Purpose |
|---|---|
| Cold-water feed valve | Connects the cold-water supply to the RO inlet. |
| Prefilters | Sediment and carbon stages condition water before the membrane. |
| RO membrane | Separates permeate from concentrate. |
| Postfilter | Final carbon stage before dispensing. |
| Automatic shut-off valve | Stops incoming flow when the system’s tank-full conditions are reached. |
| Check valve | Helps prevent treated water from flowing backward. |
| Flow restrictor | Regulates concentrate flow and supports the membrane’s operating conditions. |
| Storage tank | Stores treated water for use between production cycles. |
| Drinking-water faucet | Dispenses treated water at the sink. |
| Drain line | Carries concentrate to the drain connection. |
What Does Reverse Osmosis Remove from Water?
A complete RO system combines different treatment methods. Understanding which component does each job helps you read product claims more confidently.
THE MEMBRANE
Dissolved Solids and Specified Contaminants
THE CARBON STAGES
Chlorine Taste and Odor
Carbon prefilters and postfilters contribute taste-and-odor treatment. Compare the stated claim, capacity and replacement schedule. Explore chlorine reduction →
THE SEDIMENT PREFILTER
Sand, Silt and Rust
Particle filtration captures suspended material. It is a different job from reducing dissolved chemicals. Understand sediment filtration →
For PFAS and other specified chemicals, check testing or certification for the particular substance and complete membrane/carbon configuration. See the full RO contaminant guide for more detail.
Check the Complete System’s Claims
NSF/ANSI 58 covers point-of-use RO systems, including TDS reduction. Additional contaminant claims are verified separately. A membrane or cartridge certification does not automatically establish every claim for an assembled system.
What does RO not automatically solve? A system’s TDS reading is not a test for every contaminant or proof of microbiological safety. Ordinary under-sink RO is not a substitute for a treatment plan designed for an unsafe source. For private wells, start with testing and appropriate source-water treatment.
Benefits and Tradeoffs of Reverse Osmosis
Why People Choose RO
- Reduces dissolved solids and specific contaminants covered by the system’s claims.
- Provides treated water at the kitchen tap for drinking and cooking.
- Can reduce reliance on purchased bottled water when it replaces what you normally buy.
What to Plan For
- A reject-water stream during production.
- Space for the assembly, any storage tank and service access.
- Ongoing filter changes, membrane replacement and model-specific sanitizing.
How Much Water Does RO Send to the Drain?
The reject-water-to-treated-water ratio varies by design and operating conditions. The current PuROTwist 4000 lists a 1:1 ratio—about one gallon to the drain per gallon of treated water under rated conditions. That corresponds to 50% recovery under those conditions. Startup and service flushing also use water.
EPA WaterSense requires labeled point-of-use RO systems to use no more than 2.3 gallons of reject water per gallon of treated water. A listed ratio alone does not establish WaterSense certification; check the exact model’s labeling and documentation.
What Affects RO Water Production and Performance?
The GPD number on a product page is a production rating under specified conditions. It is not the same as the amount of water you can dispense at one time. Compare three different measurements:
Production
Gallons per day
How much treated water the membrane can make under its rating conditions.
Storage
Usable tank water
How much treated water is available between refill cycles; tank volume and drawdown differ.
Dispensing
Faucet flow
How quickly water comes out when you open the tap.
Check Feed-Water Conditions
Pressure and temperature: low feed pressure and cold water can reduce production and lengthen tank-refill time. A compatible booster pump may help when pressure is below the model’s required range.
Water chemistry: TDS, hardness, iron, sediment and other source-water conditions can affect suitability and maintenance. Carbon pretreatment helps protect chlorine-sensitive membranes; additional treatment may be needed for other conditions.
Read the exact model’s operating limits and test conditions. Pressure, temperature and water-chemistry limits are specific to the membrane and system. Use the owner’s manual and current performance sheet for your unit.
RO Performance Terms in Plain Language
- Permeate: water that passes through the RO membrane.
- Concentrate: the reject-water stream carrying material rejected by the membrane.
- TDS: total dissolved solids; a useful performance measure, not a test for every contaminant.
- GPD: gallons per day, used for rated production.
- Rejection or reduction: how much of a specified substance is reduced under the stated conditions.
- Recovery: the share of incoming water that becomes treated water; it describes water use, not the contaminant-removal percentage.
- PSI: pounds per square inch, the pressure unit used in system specifications.
Is RO the Right Treatment for Your Home?
Choose the treatment that matches your water concern. RO is useful when reducing dissolved solids or a specific documented contaminant is the goal; other methods serve different needs.
| Treatment | Often used for | Key distinction |
|---|---|---|
| Under-sink RO | Reducing dissolved solids and model-specific dissolved contaminants. | Produces concentrate; complete-system claims and upkeep matter. |
| Carbon filtration | Chlorine taste and odor and specified chemical concerns. | Generally does not reduce dissolved salts like RO. |
| Sediment filtration | Capturing sand, silt and rust particles. | Particle filtration does not address dissolved contaminants. |
| Ultrafiltration | Capturing fine suspended material while retaining dissolved minerals. | Generally does not reduce TDS the way RO does. |
For a practical shopping checklist, see what to look for when buying an RO system. Our RO buyer’s guide explains the broader options.
Compare Under-Sink RO Systems
These are complete kitchen drinking-water systems—not retrofit assemblies. Compare the priorities below, then check current dimensions, water requirements, included parts and pricing on the product page.
Microline TFC-400-ESP
A dependable kitchen RO workhorse
The Microline TFC-400-ESP brings four-stage filtration to your kitchen in one complete package: filters, a membrane rated for up to 50 GPD, a storage tank, a chrome air-gap faucet and an installation kit. It runs on household water pressure—no electricity required—and offers readily available replacement-filter kits for ongoing care.
AXEON Solo I
50 GPD · Four stages
A compact all-in-one filter assembly for shoppers comparing space-saving options; allow room for the tank and service.
PuROTwist 4000
75 GPD · Four stages · Listed 1:1 ratio
Quick-change cartridges and lower listed reject-water use under rated conditions.
Find the Correct RO Replacement Filters
Already own an RO system? Start with the brand and model to find its filters, membrane and other compatible parts. Keep the model number and manual with your service records.
FOR CURRENT RO OWNERS
Find Parts for Your RO System
Select your brand...
Select your model...
Not sure which model you own?
Recognize a Conventional Housing Layout?
These photos can help you identify a starting category. Confirm the brand, model and specified part number before ordering; matching the number of housings alone is not enough.
Keep Suitable Parts When You Upgrade
If proprietary filters are costly or difficult to find, a compatible RO conversion kit may let you replace the treatment assembly while retaining a suitable tank and faucet. Check the named system match and tubing requirements for the exact kit.
RO Maintenance at a Glance
A simple service schedule for the filters and membrane in a traditional under-sink RO system.
6–12
MONTHS
Sediment + Carbon Prefilters
Replace the specified cartridges to support flow and membrane protection.
6–12
MONTHS
Post-Carbon Filter
Replace the final polishing filter to maintain its taste-and-odor treatment.
~24
MONTHS
RO Membrane
Follow the model’s service interval and performance checks. Some conditions can require earlier replacement.
These are general guidelines. Follow the replacement schedule in your specific RO system’s owner’s manual and cartridge instructions. Water quality and use may require earlier service.
Include Sanitizing and Inspection
Use the procedure approved for your system and sanitizer. Service is also a time to inspect housings, fittings, tubing, tank operation and leaks. Choose the correct sanitizing procedure →
Signs Your RO System May Need Attention
- Faucet flow becomes noticeably slower.
- The storage tank is not filling as expected.
- Taste or odor changes.
- A TDS comparison shows declining membrane performance.
- Water keeps flowing to the drain after the tank should be full.
- A housing, fitting or tube shows moisture or leakage.
Diagnose the cause before ordering parts. Use the RO troubleshooting guide or full maintenance guide for next steps.
Where Is Reverse Osmosis Used?
One drinking-water tap, an entire property or a business: the application determines the system design.
AT HOME
ONE TAP
At the Kitchen Sink: Point of Use
A point-of-use (POU) RO system treats water at a specific drinking-water outlet. Most home units sit under the kitchen sink and supply a dedicated faucet. Some can also connect to a refrigerator dispenser or ice maker when available pressure and system instructions permit it.
WHOLE PROPERTY
POINT OF ENTRY
Throughout a Property: Point of Entry
A whole-house, or point-of-entry (POE), RO installation treats water for the wider property. This requires a different design for demand, storage, pretreatment and any supporting post-treatment. A household that mainly wants treated drinking and cooking water may be better served by point-of-use RO. Compare whole-house and under-sink RO needs →
FOR BUSINESS
PROCESS + DEMAND
Commercial and Industrial Applications
RO technology is also used in food and beverage operations, agriculture, boiler-feed treatment, hotels, pharmaceutical processes, marine applications and desalination. Requirements vary widely: these are applications of RO technology, not claims that a household unit suits each one.
Higher-demand installations can require different pretreatment, pumping, storage and controls. For example, the AXEON HYDRO 750 is a wall-mounted light-commercial option rated for up to 750 GPD under its specified conditions and requires electrical power. Explore commercial RO systems, or call our team to discuss the application.
Reverse Osmosis Frequently Asked Questions
What is the difference between an RO membrane and an RO system?
The membrane performs the separation of water from many dissolved substances. A complete RO system includes supporting filters, valves, tubing and dispensing components. A tank-based system also includes storage. Choose by the complete configuration’s documented performance.
Does RO water still contain minerals?
RO reduces many dissolved minerals and salts; the amount remaining depends on the incoming water and system. Some systems add a remineralization filter to adjust taste and pH. RO water does not have one universal mineral content.
Does a reverse osmosis system need electricity?
Conventional tank-based under-sink systems such as the examples above use household water pressure and do not require electricity. Models with electric pumps or powered features require a suitable power connection.
How much water can an RO system produce?
GPD is a daily production rating under specified conditions. It is different from faucet flow or the amount of water available in a tank at one time. Pressure, water temperature, incoming water and membrane condition affect actual production and refill time.
Can RO supply a refrigerator or ice maker?
Many under-sink systems can when their specifications, available pressure and installation allow it. Check the manufacturer’s instructions and our RO-to-refrigerator connection guide before adding a line.
Can I install an under-sink RO system myself?
Some homeowners can install a suitable system using its instructions. Check cabinet space, faucet mounting, feed connection and drain requirements first. Plumbing changes or unfamiliar work may call for a qualified installer. See the RO installation guide.
Is RO the same as bottled or distilled water?
No. Reverse osmosis is a treatment process; bottled water is a packaged product that may use different sources and treatment methods. Distillation uses evaporation and condensation. The water’s properties depend on the source and process, so compare documented treatment rather than assuming these labels mean the same thing.
Do I need whole-house reverse osmosis?
Most household drinking-water RO systems treat water at a dedicated kitchen faucet. Whole-house RO serves a different need and requires a separate design for the source water, peak demand, storage and supporting treatment. Compare RO system types before choosing.
How long does a reverse osmosis system last?
Housings, fittings, the faucet and storage tank can remain in service for many years when installed and maintained correctly. Service life depends on the model, conditions and use. Filters and membranes are consumables; replacing them is part of ownership rather than a reason to replace the whole system.
Is a reverse osmosis system noisy?
Many pressure-driven under-sink systems operate quietly, although you may hear concentrate flowing to the drain. An air-gap faucet may gurgle while the tank refills. Persistent or changed sounds should be checked against the manual and operating conditions. Understand air-gap and non-air-gap faucets or use our troubleshooting guide.
Does reverse osmosis remove fluoride?
An RO membrane can reduce fluoride, but the actual reduction depends on the system and conditions. Choose the complete model’s documented fluoride-reduction claim for your goal, rather than a universal percentage. Read the fluoride treatment guide.
Which part of an RO system reduces chlorine?
The carbon prefilter is the main stage used for chlorine taste-and-odor treatment in a typical RO system, and it also helps protect chlorine-sensitive membranes. The membrane and carbon stages have different roles. Maintain the specified carbon cartridges on schedule.
Is an RO system worth the cost?
Compare the system and installation price, filters, membranes, service and water use with what you currently spend on drinking water. It may offer savings when it replaces purchased bottled or delivered water, but savings depend on your household. Treatment fit and convenience matter alongside cost. Use the RO buying checklist to compare ownership costs.
How often should RO filters and membranes be changed?
Many traditional systems use 6–12-month prefilter intervals, an annual postfilter change and a membrane interval around 24 months. These are general references; follow the exact model and cartridge instructions and replace sooner when conditions require it. See how filter life varies.
Shop RO Systems and Replacement Parts
Compare under-sink RO systems for your home, or find the filters, membrane and parts that fit the system you own.
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